Within this application several publications are referenced by Arabic numerals within parentheses. Full citations for these references may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entirety are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.
The advent of recombinant DNA techniques has made it possible to manipulate the naturally occurring DNA sequences within an organism (the genome) in order to change in some manner the functions of the organism through genetic engineering. The present invention concerns organisms defined as viruses that infect animals and contain DNA as their genetic material; specifically viruses belonging to the herpesvirus group (herpesviruses) (23). This group of viruses comprise a number of pathogenic agents that infect and cause disease in a number of target species: swine, cattle, chickens, horses, dogs, cats, etc. Each herpesvirus is specific for its host species, but they are all related in the structure of their genomes, their mode of replication, and to some extent in the pathology they cause in the host animal and in the mechanism of the host immune response to the virus infection.
The types of genetic engineering that have been performed on these herpesviruses consist of cloning parts of the virus DNA into plasmids in bacteria, reconstructing the virus DNA while in the cloned state so that the DNA contains deletions of certain sequences, and furthermore adding foreign DNA sequences either in place of the deletions or at sites removed from the deletions. The usual method is to make insertions of the foreign DNA into the viral sequences, although the foreign DNA could be attached to the end of the viral DNA as well. One utility of the addition of foreign sequences is achieved when the foreign sequence encodes a foreign protein that is expressed during viral infection of the animal. A virus with these characteristics is referred to as a vector, because it becomes a living vector which will carry and express the foreign protein in the animal. In effect it becomes an elaborate delivery system for the foreign protein.
The prior art for this invention stems first from the ability to clone and analyze DNA while in the bacterial plasmids. The techniques that are available for the most part are detailed in Maniatis et al. (1). This publication gives state-of-the-art general recombinant DNA techniques.
The application of recombinant DNA techniques to animal viruses has a relatively recent history from about 1980. The first viruses to be engineered have been the smallest ones--the papovaviruses. These viruses contain 3000-4000 base pairs (bp) of DNA in their genome. Their small size makes analysis of their genomes relatively easy and in fact most of the ones studied (SV40, polyoma, bovine papilloma) have been entirely sequenced. Because these virus particles are small and cannot accommodate much extra DNA, and because their DNA is tightly packed with essential sequences (that is, sequences required for replication), it has not been possible to engineer these viruses as live vectors for foreign gene expression. Their entire use in genetic engineering has been as defective replicons for the expression of foreign genes in animal cells in culture (roughly analogous to plasmids in bacterial systems) or to their use in mixed populations of virions in which wild type virus acts as a helper for the virus that has replaced an essential piece of DNA with a foreign gene. The studies on papovaviruses do not suggest or teach the concept of living virus vectors as delivery systems for host animals.
The next largest DNA animal viruses are the adenoviruses. In these viruses there is a small amount of nonessential DNA that can be replaced by foreign sequences. The only foreign genes that seem to have been expressed in adenoviruses are the T-antigen genes from papovaviruses (2,3,4,5), and the herpes simplex virus thymidine kinase gene (28). It is possible, given this initial success, to envision the insertion of other small foreign genes into adenoviruses. However the techniques used in adenoviruses do not teach how to obtain the same result with herpesviruses. In particular, these results do not identify the nonessential regions in herpesviruses wherein foreign DNA can be inserted, nor do they teach how to achieve the expression of the foreign genes in herpesviruses, e.g. which promoter signals and termination signals to use.
Another group of animal viruses that have been engineered are the poxviruses. One member of this group, vaccinia, has been the subject of much research on foreign gene expression. Poxviruses are large DNA-containing viruses that replicate in the cytoplasm of infected cells. They have a structure that is very unique among viruses--they do not contain any capsid that is based upon icosahedral symmetry or helical symmetry. In theorizing on the origin of viruses, the poxviruses are the most likely ones to have originated from bacterial-like microorganisms through the loss of function and degeneration. In part due to this uniqueness, the advances made in the genetic engineering of poxviruses cannot be directly extrapolated to other vital systems, including herpesviruses. Vaccinia recombinant virus constructs have been made in a number of laboratories that express the following inserted foreign genes: herpes simplex virus thymidine kinase gene (6,7), hepatitis B surface antigen (8,9,29), herpes simplex virus glycoprotein D gene (8,29), influenza hemagglutinin gene (10, 11), malaria antigen gene (12), and vesicular stomatitis glycoprotein G gene (13). The general overall features of the vaccinia recombinant DNA work are similar to the techniques used for all the viruses, especially as they relate to the techniques in reference (1). However in detail, the vaccinia techniques do not teach how to engineer herpesviruses.
Vaccinia DNA is not infectious, so the incorporation of foreign DNA must involve an infection/transfection step that is not appropriate to other viruses, and vaccinia has unique stability characteristics that make screening easier. In addition, the signal sequence used by promoters in vaccinia are unique and will not work in other viruses. The utility of vaccinia as a vaccine vector is in question because of its close relationship to human smallpox and its known pathogenicity to humans. The use of host-specific herpesviruses promises to be a better solution to animal vaccination.
Herpesviruses contain 100,000 to 150,000 base pairs of DNA as their genetic material, and several areas of the genome have been identified that are dispensible for the replication of virus in vitro in cell culture. Modifications of these regions of the DNA are known to lower the pathogenicity of the virus, i.e. to attenuate the virus, for an animal species. For example, inactivation of the thymidine kinase gene renders human herpes simplex virus non-pathogenic (45), and pseudorabies virus of swine non-pathogenic (46 and 47).
Removal of part of the repeat region renders human herpes simplex virus non-pathogenic (48 and 49). A repeat region has been identified in Marek's disease virus that is associated with viral oncogenicity (50). A region in herpesvirus saimiri has similarly been correlated with oncogenicity (51). However, modifications in these repeat regions do not teach the construction of attenuated pseudorabies viruses with deletions in repeat sequences.
The degree of attenuation of a virus is important in the utility of the virus as a vaccine. Deletions which cause too much attenuation of the virus will result in a vaccine that fails to elicit an adequate immune response.
The herpesviruses are known to cause a variety of latent and recurrent infections in human and other vertebrates and are even known to infect a fungus and an oyster. Among the conditions associated with herpesvirus infections are fever blisters caused by herpes simplex type 1, genital herpes causes by herpes simplex type 2, and chickenpox in children and shingles in adults cause by herpes zoster infection. Pseudorabies virus (PRV), a Class D herpesvirus, induces Aujesky's disease, an acute and often fatal nervous condition, in domestic and wild animals.
Among the herpesviruses, only herpes simplex of humans and, to a limited extent, herpes saimiri of monkeys have been engineered to contain foreign DNA sequences previous to this disclosure. The earliest work on the genetic manipulation of herpes simplex virus involved the rescue of temperature sensitive mutants of the virus using purified restriction fragments of DNA (14). This work did not involve cloning of the DNA fragments into the viral genome. The first use of recombinant DNA to manipulate herpes simplex virus involved cloning a piece of DNA from the L-S junction region into the unique long region of the DNA, specifically into the thymidine kinase gene (15). This insert was not a foreign piece of DNA, rather it was a naturally occurring piece of herpesvirus DNA that was duplicated at another place in the genome. This piece of DNA was not engineered to specifically express any protein, and thus it did not teach how to express protein in herpesviruses. The manipulation of herpes simplex next involved the creation of deletions in the virus genome by a combination of recombinant DNA and thymidine kinase selection. The first step was to make a specific deletion of the thymidine kinase gene (16). The next step involved the insertion of the thymidine kinase gene into the genome at a specific site, and then the thymidine kinase gene and the flanking DNA at the new site were deleted by a selection against thymidine kinase (17). In this manner herpes simplex apha-22 gene has been deleted (17). In the most recent refinement of this technique, a 15,000 bp sequence of DNA has been deleted from the internal repeat of herpes simplex virus (18).
The insertion of genes that encode protein into primate herpesviruses have involved seven cases: the insertion of herpes simplex glycoprotein C back into a naturally occurring deletion mutant of this gene in herpes simplex virus (19); the insertion of glycoprotein D of herpes simplex type 2 into herpes simplex type 1 (20), again with no manipulation of promoters since the gene is not really `foreign`; the insertion of hepatitis B surface antigen into herpes simplex virus under the control of the herpes simplex ICP4 promoter (21); and the insertion of bovine growth hormone into herpes saimiri virus with an SV40 promoter that in fact didn't work in that system (an endogenous upstream promoter served to transcribe the gene) (22). Two additional cases of foreign genes (chicken ovalbumin gene and Epstein-Barr virus nuclear antigen) have been inserted into herpes simplex virus (30), and glycoprotein X of pseudorabies virus has been inserted into herpes simplex virus (31).
These limited cases of deletion and insertion of genes into herpesviruses demonstrate that it is possible to genetically engineer herpesvirus genomes by recombinant DNA techniques. The methods that have been used to insert genes involve homologous recombination between the vital DNA cloned on plasmids and purified viral DNA transfected into the same animal cell. In aggregate this is referred to as the homologous recombination technique. This technique with minor modifications has been adaptable to other herpesviruses that we have engineered. However, the extent to which one can generalize the location of the deletion and the sites for insertion of foreign genes is not obvious from these previous studies. Furthermore, it is also not obvious that non-primate herpesviruses are amenable to the same techniques as the primate herpesviruses, and that one could establish a targeted approach to the deletion, insertion, and expression of foreign genes.
Infectious bovine rhinotracheitis (IBR) virus, an alpha-herpesvirus with a class D genome, is an important pathogen of cattle. It has been associated with respiratory, ocular, reproductive, central nervous system, enteric, neonatal and dermal diseases (37). Cattle are the normal hosts of IBR virus, however it also infects goats, swine, water buffalo, wildebeest, mink and ferrets. Experimental infections have been established in muledeer, goats, swine, ferrets and rabbits (38).
Conventional modified live virus vaccines have been widely used to control diseases caused by IBR. These vaccine viruses may revert to virulence, however. More recently, killed virus IBR vaccines have been used, but their efficacy appears to be marginal.
IBR has been analyzed at the molecular level as reviewed in (39). A restriction map of the genome is available in this reference, which will aid in the genetic engineering of IBR according to the methods provided by the present invention. No evidence has been presented that IBR has been engineered to contain a deletion or an insertion of foreign DNA.
Marek's disease virus (MDV) causes fowl paralysis, a common lymphoproliferative disease of chickens. The disease occurs most commonly in young chickens between 2 and 5 months of age. The prominant clinical signs are progressive paralysis of one or more of the extremeties, incoordination due to paralysis of legs, drooping of the limb due to wing involvement, and a lowered head position due to involvement of the neck muscles. In acute cases, severe depression may result. In the case of highly oncogenic strains, there is characteristic bursal and thymic atrophy. In addition, there are lymphoid tumors affecting the gonads, lungs, liver, spleen, kidney and thymus (37).
All chicks are vaccinated against MDV at one day of age to protect the chick against MDV for its lifetime. One vaccine method for MDV involves using turkey herpesvirus (HVT). It would be advantageous to incorporate other antigens into this vaccination at one day of age, but efforts to combine vaccines have not proven satisfactory to date due to competition and immunosuppression between pathogens. The multivalent vaccines engineered in this invention are a novel way to simultaneously vaccinate against a number of different pathogens.
A restriction map of both MDV (43) and HVT (34) are available in the literature. There is no evidence to suggest that anyone has successfully created a deletion or insertion of foreign DNA into MDV or HVT prior to this disclosure.
Other herpesviruses contemplated to be amenable to these procedures are feline herpesvirus (FHV), equine herpesvirus (EHV), and canine herpesvirus (CHV). These pathogens cause disease in each of their respective hosts. Feline herpesvirus causes feline rhinotracheitis, an acute upper respiratory tract infection characterized by fever, pronounced sneezing, nasal and lacrimal secretions, and depression. The virus may cause corneal ulceration and abortion. The nasal passages and turbinates show focal necrosis, and the tonsils are enlarged and hemorrhagic. Equine herpesvirus causes rhinopneumonitis, abortion, exanthema of the genitals and occasionally neurologic disease. The acute disease is characterized by fever, anorexia and a profuse, serous nasal discharge. The neurologic symptoms, when present, consist of ataxia, weakness and paralysis. Canine herpesvirus causes severe illness in young puppies, where mortality may reach 80%. The disease is characterized by viremia, anorexia, respiratory illness, abdominal pain, vomiting and incessant crying. Generally, there is no fever. The principal lesions are disseminated necrosis and hemorrhages in the kidneys, liver and lungs.
The molecular biology of the feline, equine and canine herpesviruses are in their initial phases. Partial restriction maps are available for equine herpesvirus, and in progress in at least one lab for the feline herpesvirus. Beyond this type of genome analysis, no evidence for the deletion or insertion of foreign genes into these viruses is available.
The present invention involves the use of genetically engineered herpesvirus to protect animals against disease. It is not obvious which deletions in herpesviruses would serve to attenuate the virus to the proper degree. Even testing vaccine candidates in animal models, e.g. mice, does not serve as a valid predictor of the safety and efficacy of the vaccine in the target animal species, e.g. swine.
Another subject of the present invention is a vaccine for pseudorabies virus (herpesvirus suis, suid herpesvirus 1, or Aujesky's disease virus) disease of swine. Swine are the natural host of pseudorabies virus in which infection in older animals is commonly inapparent but may be characterized by fever, convulsions, and death particularly in younger animals. Pseudorabies also infects cattle, sheep, dogs, cats, ferrets, foxes, and rats (37) where the infection usually results in death. Death is usually preceded by intense pruritus, mania, encephalitis, paralysis, and coma. Traditional live vaccines are available for use in swine, but they are lethal for the other animals. An improved vaccine for pseudorabies would induce a more reliable immune response in swine, would be specifically attenuated to be incapable of reversion to virulence, and would not cause disease in other hosts.
Pseudorabies virus, an alpha-herpesvirus of swine, has a genome of class D (23); that is it contains two copies of a single repeat region, one located between the unique long and unique short DNA region and one at the terminus of the unique short region (see FIG. 1). Herpes simplex virus is an alpha-herpesvirus with a class E genome (23); that is it contains two copies of each of two repeats. Herpes saimiri is a gamma-herpes-virus with a class B genome; that is, it contains numerous reiterations of the same sequence at both termini (23). As the genome structure differs significantly between these different classes of herpesviruses, and because the different viruses attack different cells within their hosts and elicit different pathologies, it is necessary in each instance to establish which specific regions can be removed in order to attenuate and which regions can be altered to express foreign genes.
Pseudorabies virus has been studied using the tools of molecular biology including the use of recombinant DNA techniques. BamHI, KpnI, and BgIII restriction maps of the virus genome have been published (24, 27). DNA transfection procedures have been utilized to rescue temperature sensitive and deletion mutants of the virus by the homologous recombination procedure (24). There are two examples of deletions that have been made in the pseudorabies virus genome--one is a thymidine kinase gene deletion (25), also disclosed in U.S. Pat. No. 4,514,497 entitled "Modified Live Pseudorabies Viruses". This patent teaches thymidine kinase deletions only and does not suggest other attenuating deletions, nor does it suggest insertion of foreign DNA sequences. The other reference involves the deletion of a small DNA sequence around a HindIII restriction site in the repeat region (26) upon which European Patent Publication No. 0141458, published on May 15, 1985, corresponding to European Patent Application No. 84201474.8, filed on Oct. 12, 1984 is based. This patent application does not teach or suggest attenuating deletions nor does it teach or suggest the insertion of DNA sequences into pseudorabies virus.
The present invention concerns deletions which have been introduced into the pseudorabies virus genome at sites previously undisclosed. Foreign DNA sequences have also been introduced into the attenuated pseudorabies virus genome and expressed as proteins. One embodiment of the invention concerns a vaccine useful for preventing pseudorabies and other swine diseases with a single inoculum.
Other relevant pseudorabies literature disclosed herein, concerns the presence of naturally-occurring deletions in the genome of two vaccine strains of pseudorabies viruses (27). These deletions are responsible, at least in part, for the attenuated nature of these vaccines however they do not occur in a repeat sequence and do not suggest the attenuation of pseudorabies virus by deleting a portion of a repeat sequence. Such naturally-occurring deletions do not teach methods for making these deletions starting with wild type pseudorabies virus DNA, nor do they suggest other locations at which to make attenuating deletions. There are no examples of naturally-occurring insertions of foreign DNA in herpesviruses.
The natural host of pseudorabies virus is swine, in which infection is commonly inapparent but may be characterized by fever, convulsions and paralysis. Pseudorabies virus also infects cattle, sheep, dogs, cats, foxes and mink, where infection usually results in death of the host. The predominant visible feature of pseudorabies viral infection is intense pruritis generally resulting in host mutilation of the involved area. Violent excitement, fits and paralysis, all symptoms of encephalomyelitis, precede death which usually occurs within a few days following onset of clinical signs.
Pseudorabies virus disease in swine is of serious concern to governmental bodies worldwide. In the United States, swine from infected herds cannot be sold except to slaughterhouses. Several individual states have separately enacted eradication control practices against pseudorabies. At the current time, any animal vaccinated for pseudorabies disease is treated as though it were infected with pseudorabies virus and is subject to the same regulatory constraints. This is due primarily to the lack of a diagnostic test to differentiate vaccinated from infected animals.
The research and development trend among traditional vaccine manufacturers has generally emphasized research leading to vaccines that are based upon virus subunits rather than live viruses. This departure from live virus vaccines is due partly to the recognized safety aspect of subunit vaccines, and their unlikelihood of containing infectious live viruses. Another reason for developing a subunit vaccine has been to allow for the development of a diagnostic test that would accompany the vaccine and would differentiate vaccinated from infected animals, thereby escaping from the regulatory burden following use of other vaccines.
Subunit vaccines also have limitations. They contain a limited number of viral antigens compared to those produced by live viruses. This paucity of antigens produces a weak immune response of short duration in the vaccinated animal at considerably greater cost than a live virus vaccination. However, the limited spectrum of antigens in the subunit vaccine allows the vaccinated swine to be distinguished from swine which have been infected with the wild-type virus. The ability to distinguish vaccinated from infected swine is a crucial property of a pseudorabies vaccine because none of the known vaccines prevent the vaccinated animals from being super-infected by the wild-type virus. While the vaccinated animals do not become sick upon super-infection, there is strong evidence that they may become carriers of the wild-type virus and pass the wild-type virus to other swine.
In any eradiciation program aimed at eliminating pseudorabies virus, a vaccine provided with characteristics which would allow vaccinated animals to be distinguished from animals infected with wild-type virus would be advantageous. The subunit vaccines have high cost and poor efficacy but an animal vaccinated with this type of vaccine will produce antibodies only to the limited spectrum of antigens present in the vaccine. By sampling the serum of the swine, it is possible to show that the vaccinated animal has antibodies only to the antigens contained in the vaccine while an animal infected with the wild-type virus would have antibodies against a wider range of antigens. A subunit vaccine used in this way to differentiate vaccinated from pseudorabies infected animals has been disclosed in European Patent Application No. 8540074.4, filed on Sep. 4, 1985, published Nov. 27, 1985 as European Publication No. 0162738 and entitled "Production of Pseudorabies Virus Subunit Vaccines". This published patent application does not teach or suggest the construction or use of a similar diagnostic test in conjunction with a live virus vaccine. The vaccination of an animal with a live virus which would result in an immune response distinguishable from wild-type infection would also have the further advantages of low cost and high efficacy associated with live virus vaccines.
Deletions in genes coding for viral antigens have been described previously. A spontaneous deletion in the glycoprotein C gene of herpes simplex virus (52), a spontaneous deletion in the glycoprotein A gene of Marek's disease virus (53) a spontaneous deletion in the glycoprotein A gene (also called glycoprotein gI) of PRV (27,55) and the absence or greatly reduced amount of glycoprotein gIII in some PRV mutants (54) are known. However, all of these deletions arose spontaneously in an uncontrolled process. Hence, it has not been possible to direct deletions to DNA encoding for specific antigens to control the deletion process and direct the deletions to antigens particularly suitable as diagnostic markers.
The presence or absence of particular antigens in any infectious disease can be exploited as a diagnostic test for the infectious disease agent. This presence or absence forms the basis for all immunolgocial diagnositc tests, which differ only in the details of their specific immunological approach. Publications pertinent to the current invention include Wathan and Wathan (54) who reported that either the gI gene or the gIII gene could be deleted from PRV and suggested that the resulting virus could be used for distinguishing vaccinated from infected swine. However, they did not describe the methodology necessary to create the vaccine, they did not demonstrate the utility of such a vaccine in serological tests and they did not in any other way prove the feasibility of such a vaccine.
Van Oirschot, et al. (56), have used a special monoclonal-based immunological detection system for gI of PRV and have shown that pigs inoculated with naturally-occuring vaccine strains which are missing at least a portion of the gI gene can be differentiated from pigs infected by wild-type PRY. However, this diagnostic test may be used for any of several vaccines against PRV that are already existing in both Europe and the U.S. without differentiating which vacccine was used. This limits the usefulness of this diagnostic, since the vaccines which are detectable have differing biological and virulence properties.
The approach of deleting a gene to attenuate a virus coupled with a diagnostic for that gene, provides a vaccine that can be differentiated from any of the currently used PRV vaccines and from wild-type PRV. It is important to be able to differentiate a new, safer vaccine from those currently used because pigs receiving the current vaccines are all regulated during eradication programs to the same extent as those infected with wild-type PRV.
Antigens of choice for the purpose of a diagnostic marker would have the following characteristics: 1) the antigens and their genes would be non-essential for the production of infectious virus in tissue culture; and 2) the antigen would elicit a major serological response in the animal, but is preferably not an important neutralizing antigen.
The present invention therefore involves the ability to attenuate pseudorabies virus of swine to create a live virus vaccine and the ability to distinguish whether an animal has been given the vaccination or whether the animal has been infected by wild-type pseudorabies virus.